Continuous casting installation comprising a soft reduction section

ABSTRACT

The invention relates to a method and a device for the continuous casting of slabs or ingots, in particular of thin slabs in a continuous casting installation. Said installation comprises a soft reduction section in a continuous casting guide under the mould. The soft reduction section contains pressure rollers and support roller ( 3, 4 ), which are continuously restrained in relation to one another, either individually or as a segment ( 1 ), by means of hydraulic cylinders ( 7, 7′ ) and are restrained in a limiting manner by stops ( 30 ). The installation uses a hard-pressure restraining force in an area of the soft reduction section that has not yet completely solidified and a soft-pressure restraining force in an area of the soft reduction section that has completely solidified. Threshold and changeover values for the hard and soft-pressure are defined in the segment is restrained using hard-pressure, such a way that if the restraining force lies below the threshold or changeover value, and if the restraining force lies above the threshold or changeover value the segment is restrained using soft-pressure.

[0001] The invention relates to a method of an apparatus for continuouscasting of slabs and ingots, in particular, of thin slabs in acontinuous casting installation including a soft reduction section in astrand guide under a mold and having pressure rollers and supportrollers which are continuously restrained relative to one another,either individually or as a segment by hydraulic cylinders, and arerestrained by stops in a limiting manner.

[0002] Many presently operating continuous casting installations andcontinuous casting installations the operation of which is to begin, inparticular, continuous casting installations for casting thin slabs usean operational technology with soft reduction. The method and acorresponding apparatus are primarily installed in the horizontalportion of a strand guide and serve for improving the microstructuralquality and the surface quality of the cast products.

[0003] A soft reduction effect is achieved by a reduction of a strandthickness in the region of its solidification with a molten core up tothe tip of a liquid crater. The region, in which the soft reductionsignificantly affects the strand inner quality lies in thesolidification range at FS: 0.3 through 0.8 according to a longitudinalposition on the solidification path. The soft reduction, which consistsin reduction of the thickness in small steps in the above-mentionedregion, prevents to a substantial extent segregation in the melt.

[0004] In order to achieve the soft reduction in a defined operationalregion at different operation conditions, which can be caused, e.g., bychanges in the casting speed, or by stoppages, the zone, in which thestrand thickness is reduced, should be changeable along the longitudinalposition of the strand. To this end, a position controlled powercylinder is used in the support segments.

[0005] In order to effect a predetermined reduction of the thickness ofa running strand, the support rollers are conically adjusted to aposition favorable for soft reduction by determining the position of thecrater and by determining, by calculations or by measurements, thechange in the pressure applied to the pressure rollers by the strand.This method is practically completely developed and is designated as adynamic soft reduction that, however, because of the used technicalmeans, is connected with high costs.

[0006] An object of the invention, proceeding from the above-mentionedstate of the art, is a method and an apparatus for casting slabs oringots, in particular, thin slabs, which is recited in the preamble ofclaim 1 and which would make possible a highly effective soft reductionwith reduced costs of the technical means and with a high quality of theresults.

[0007] In order to achieve the above-mentioned object, according to theinvention, hard-pressure restraining force is used in an area of thesoft reduction section that has not yet completely solidified, and asoft-pressure restraining force is used in an area of the soft reductionsection that has completely solidified, with a threshold or changeovervalue for the hard-pressure and the soft-pressure being defined and witha restraining force being below the threshold or changeover value,restraining of the segment is effected by using the hard-pressure, andabove the threshold or changeover value, restraining of the segment iseffected by using the soft pressure.

[0008] The inventive process is based on the knowledge obtained from apractical operation that a completely solidified section of a strand, incase it reaches the region of the soft reduction, should not be deformedas, otherwise, the support or pressure rollers, or their bearings, orthe segment frame would be damaged, in particular when the sortreduction region is formed of several conically adjustable segments.Such a case can exist when the casting speed is changed and/or when theoperation is interrupted. In such a case, the restraining force for thesoft reduction must be able to change from the hard-pressure to thesoft-pressure, amounting advantageously to 40% of the hard pressure inrun-out section of the strand.

[0009] According to the invention, this changeover from thehard-pressure to the soft-pressure and, vice versa, if needed, takesplace automatically as a result of determination of a pressure increasepulse produced in the power cylinder upon entrance of non-deformable,completely solidified strand in the conically adjusted segment.

[0010] According to one embodiment of the method, it is contemplatedthat at a running cast operation with the hard-pressure in the softreduction section, upon reaching an upper threshold or changeover valueof the restraining force, a changeover to the soft-pressure take placeand, vice versa, at a running cast operation with the soft pressure inthe soft reduction section, upon reaching a lower threshold orchangeover value for the restraining force, a changeover to thehard-pressure takes place.

[0011] According to a further development of the inventive method, uponentry of a non-deformable, completely solidified strand section in aconically adjusted, for a hard pressure, segment, a changeover from ahard pressure to a soft pressure takes place, based on a resultingpressure increase pulse in the power cylinder.

[0012] The power cylinders are primarily subjected to a soft reductionpressure, a so-called hard-pressure, with each displacement of thepiston resulting in a pressure change. A pressure sensor on the cylinderprovides for the use of the pressure increase in the cylinder forswitching to a soft-pressure. For practically effecting this, a shut-offvalve is partially open by a relief valve so that a segment in thehydraulic switching circuit can give way, i.e., the segment inlet widthchanges so that the completely solidified strand section can passthrough the segment at the soft-pressure.

[0013] According to a further embodiment of the inventive method, uponswitching of a segment to an operation condition with a soft-pressurefor a completely solidified strand section in the segment, a pressureinquiry is initiated wherein in short, cyclic, time intervals,changeover from the soft pressure to the hard pressure and vice versatakes place, and it is checked whether the pressures remain unchanged orwhether they are above or below a respective threshold or changeovervalue and, dependent on a pressure test an a adjustment to the hardpressure or the soft-pressure takes place. These measures prevent afurther opening of the segment by a ferrostatic pressure of the melt.

[0014] According to a still further embodiment of the inventive method,all segments in the soft reduction region and the segments behind themup to machine end are subjected to a cyclic pressure inquiry. Thereby, auniform quality of a cast product is insured.

[0015] According to a preferred embodiment of the method, in particularin a horizontal portion of a continuous casting installation, a strandsection, in which soft reduction conditions are true for an operationstate of the casting operation and for used steel types, is determined.Then, in the determined strand support section, all support segments areindividually adjusted with a necessary conicity in such a way that arunning-through strand retains a necessary for the soft reduction,deformation.

[0016] To this end, the hydraulically adjustable support segments areindividually adjusted, with respect to a thickness predetermined by amechanical stop, to a given amount of soft reduction. The mechanicalstops prevent the support rollers from approaching to close to eachother and thereby any damage of the support structure.

[0017] An apparatus for continuous casting of slabs and ingots, inparticular, of thin slabs in a continuous casting installation includinga soft reduction section in a strand guide under a mold and havingpressure rollers and support roller which are continuously restrainedrelative to one another, either individually or as a segment byhydraulic cylinders, and are restrained by stops in a limiting manner,includes a segment with support and pressure rollers adapted to astrand, driven with an adjustable speed, and arranged in a segment framewith frame upper part and frame lower part, and a hydraulic cylinder.According to the invention, a hydraulic circuit includes a pressuresensor and a pressure changeover valve operated by it, a shut-off valvearranged in its switching circuit, a control valve and a relief valve, apipe-break preventing means including a pressure relief valve with acheck valve. The apparatus serves to provide a hard-pressure restrainingforce within the not yet completely solidified region of the softreduction section or a soft-pressure restraining force within thecompletely solidified region of the soft reduction section and forswitching from the hard pressure to the soft-pressure and vice versa.The restraining force is uniformly led into the pressure and supportrollers from the hydraulic cylinder through the frame upper part and theframe lower part.

[0018] Further particularities, features, and advantages of theinvention will become apparent from the following description of anembodiment of the invention shown schematically in the drawing. Thedrawings show:

[0019]FIG. 1 a section of a support segment for a cast strand, togetherwith an associated circuit diagram for effecting the method according tothe invention;

[0020]FIG. 2 a support segment in an open position for running in of astrand with a partially liquid core in the region of the tip of thecrater.

[0021]FIG. 1 shows a section of a support segment 1 for a strand 2,e.g., of a thin slab cast in a continuous casting installation. Thesupport segment is located within a soft reduction section of a strandguide and has pressure rollers 3 and support rollers 4 which arecontinuously restrained relative to one another, either individually or,in the case considered here, as a segment, with a hydraulic cylinder 7.

[0022] The numeral 30 designates a mechanical stop which defines apredetermined thickness and against which a segment lower part 6 can beindividually adjusted relative a segment upper part 5 to an amount ofsoft reduction by pulling the piston rod of the cylinder 7.

[0023] In the soft reduction section, the thickness of the cast thinslab is reduced in small steps in order to prevent segregation withinthe strand and in order to improve the surface quality. The softreduction section includes at least large elongate regions of the strandwith molten core tip to the tip of the liquid crater. In order to adaptthe adjustment of the support segments to a respective changing positionof the crater tip, which is changed as a result of different operationalconditions, a special hydraulic circuit is provided.

[0024] As shown in the circuit diagram of FIG. 1, the working pressureof the hydraulic circuit is tapped above and beneath the piston of thehydraulic cylinder 7 and is evaluated by a pressure sensor 10. Thepressure sensor 10 on the cylinder 7 provides for use of the pressureincrease in the cylinder, e.g., for switching from hard-pressure tosoft-pressure. To this end, the pressure sensor 10 is connected with apressure changeover valve 11 by signal lines.

[0025] The power cylinder 7 is subjected to the action of, primarily,the soft reduction pressure, i.e., to the action of hard-pressure, andis connected, by a shut-off valve 12, e.g., a pipe-break valve or apilot-controlled check valve, with a pressure relief valve 15 in such away that each displacement of the piston results in a pressure changewhich, as it has already been mentioned above, in case of a pressureincrease in the cylinder, is used for switching to a soft pressure.

[0026] In case of soft pressure, on the other hand, the shut-off valve12 is open by a relief valve 14, and the segment can give in, i.e., thesegment inlet width can be changed, whereby the solidified strand 2 canpass through the segment 1 under soft pressure.

[0027] When at another point in time, a non-solidified strand regionwith a fluid crater reaches the segment 1, the segment 1 is subjected toan action of soft pressure and gives further in under the action of theferrostatic pressure of the strand 2, opening even more, which is notacceptable and which causes a reduction in quality of the cast strand.

[0028] In order to prevent a further opening of the segment 1 under theferrostatic pressure of the non-solidified strand 2, a pressure inquiryin initiated in the hydraulic circuit, at which in short time intervalswith time pulses, the control valve is switched, by the pressure sensor10, from soft pressure to hard pressure, and the check valve 12 isclosed.

[0029] Finally, it is checked whether the hard-pressure remainsconstant, i.e., whether it remains under the changeover valve or exceedsit. If the pressure is below the changeover value, it means that thestrand 2 has not yet completely solidified and a hard pressure is neededfor restraining the segment.

[0030] When, however, the pressure exceeds the changeover value, itmeans that the strand 2 has completely solidified, and the segmentrestrain should be effected with soft pressure.

[0031]FIG. 2 shows a segment 1 in a position for running of the strand2, with strand shells 2′ not yet closed with a core 8 of liquid melt,in. The segment frames include the segment frame-upper part 5 and thesegment frame-lower part 6 which are clamped together with a relativelylarge force against the action of the ferrostatic pressure of the strandshells 2′ and the liquid molter core 8. The segment 1 is equipped withpressure guide rollers 3 and 4 which are partially provided with drives,as is known.

[0032] The hydraulic cylinder 7′ is subjected to the action of the softpressure, and the hydraulic cylinder 7 is subjected to the action of thehard-pressure, in accordance with the present invention.

1. (Amended) A method of continuous casting of slabs and ingots, inparticular, of thin slabs in a continuous casting installation includinga soft reduction section in a strand guide under a mold and havingpressure rollers and support rollers (3,4) which are continuouslyrestrained relative to one another, either individually or as a segment(1) by hydraulic cylinders (7, 7′), and are restrained by stops (30) ina limiting manner, characterized in that a hard-pressure restrainingforce is used in an area of the soft reduction section in which thestrand has not yet completely solidified, and a soft-pressurerestraining force is used in an area of the soft reduction section inwhich the strand has completely solidified, and in that a threshold orchangeover value for the hard-pressure and the soft-pressure is defined,and that at a restraining force below the threshold or changeover value,restraining of the segment is effected by using the hard-pressure, andabove the threshold or changeover value, restraining of the segment iseffected by using the soft pressure.
 3. (Amended) A method according toclaim 1, characterized in that upon entry of a non-deformable,completely solidified strand section in a conically adjusted, for a hardpressure, segment, a changeover from a hard-pressure to a soft-pressuretakes place, based on a resulting pressure increase pulse in the powercylinder.
 4. (Amended) A method according to claim 1, characterized inthat upon switching of a segment to an operational conditional with asoft-pressure for a completely solidified strand section, in case of afollowing feeding of a non-completely solidified strand section in thesegment; a pressure inquiry is initiated, wherein in short, cyclic, timeintervals, a changeover from the soft-pressure to the hard-pressure andvice versa takes place, and it is checked whether the pressures remainunchanged or whether they are above or below a respective threshold orchangeover value and, dependent on a pressure test an adjustment to thehard pressure or the soft pressure takes place.
 6. (Amended) A methodaccording to claim 1, characterized in that in particular in ahorizontal portion of the continuous casting installation, a strandsection, in which soft reduction conditions are true for an operationalstate of the casting operation and for used steel types, is determined.8. (Amended) A method according to claim 1, characterized in that thehydraulically adjustable support segments are individually adjusted,with respect to a thickness predetermined by a mechanical stop (30), toa given amount of soft reduction.
 9. (Amended) A method according toclaim 1, characterized in that the soft-pressure is adjusted to aboutbetween 30% and 50% of the hard-pressure, e.g., to 40% of thehard-pressure.
 10. (Amended) An apparatus for continuous casting ofslabs and ingots, in particular, of thin slabs in a continuous castinginstallation including a soft reduction section in a strand guide undera mold and having pressure rollers and support rollers (3,4) which arecontinuously restrained relative to one another, either individually oras a segment (1) by hydraulic cylinders (7,7′) with a change of therestraining pressure, and are restrained by stops (30) in a limitingmanner, in particular for effecting the method according to theinvention, characterized by a hydraulic circuit with a pressure sensor(10) and a pressure changeover valve (11) operated by it, a shut-offvalve (12) arranged in its switching circuit, a control valve (13) and arelief valve, a pipe-break preventing means including a pressure reliefvalve (15) with a check valve for providing a hard-pressure restrainingforce within an area of the soft reduction section in which the strandhas not yet completely solidified, or a soft-pressure restraining forcewithin an area of the soft reduction section in which the strand hascompletely solidified, and for switching from the hard pressure to thesoft pressure and vice versa.
 1. A method of continuous casting of slabsand ingots, in particular, of thin slabs in a continuous castinginstallation including a soft reduction section in a strand guide undera mold and having pressure rollers and support rollers (3,4) which arecontinuously restrained relative to one another, either individually oras a segment (1) by hydraulic cylinders (7, 7′), and are restrained bystops (30) in a limiting manner, characterized in that a hard-pressurerestraining force is used in an area of the soft reduction section thathas not yet completely solidified, and a soft-pressure restraining forceis used in an area of the soft reduction section that has completelysolidified, and in that a threshold or changeover value for thehard-pressure and the soft-pressure is defined, and that at arestraining force below the threshold or changeover value, restrainingof the segment is effected by using the hard-pressure, and above thethreshold or changeover value, restraining of the segment is effected byusing the soft pressure.
 2. A method according to claim 1, characterizedin that at a running cast operation with the hard pressure in the softreduction section, upon reaching an upper threshold or changeover valueof the restraining force, a changeover to the soft pressure takes placeand, vice versa, at a running cast operation with the soft pressure inthe soft reduction section, upon reaching a lower threshold orchangeover value for the restraining force, a changeover to thehard-pressure takes place.
 3. A method according to claim 1 or 2,characterized in that. upon entry of a non-deformable, completelysolidified strand section in a conically adjusted, for a hard pressure,segment, a changeover from a hard-pressure to a soft-pressure takesplace, based on a resulting pressure increase pulse in the powercylinder.
 4. A method according to one or more of claims 1 through 3,characterized in that upon switching of a segment to an operationalconditional with a soft-pressure for a completely solidified strandsection, in case of a following feeding of a non-completely solidifiedstrand section in the segment; a pressure inquiry is initiated, whereinin short, cyclic, time intervals, a changeover from the soft-pressure tothe hard-pressure and vice versa takes place, and it is checked whetherthe pressures remain unchanged or whether they are above or below arespective threshold or changeover value and, dependent on a pressuretest an adjustment to the hard pressure or the soft pressure takesplace.
 5. A method according to claim 4, characterized in that allsegments in the soft reduction region and segments behind them up to amachine end are subjected to the cyclic pressure inquiry.
 6. A methodaccording to one or more of claims 1 through 5, characterized in that inparticular in a horizontal portion of the continuous castinginstallation, a strand section, in which soft reduction conditions aretrue for an operational state of the casting operation and for usedsteel types, in determined.
 7. A method according to claim 6,characterized in that in the determined strand support section, allsupport segments are individually adjusted with a necessary conicity insuch a way that a running-through strand retains a necessary for thesoft reduction, deformation.
 8. A method according to one or more ofclaims 1 through 7, characterized in that the hydraulically adjustablesupport segments are individually adjusted, with respect to a thicknesspredetermined by a mechanical stop (30), to a given amount of softreduction.
 9. A method according to one or more of claims 1 through 8,characterized in that the soft-pressure is adjusted to about between 30%and 50% of the hard-pressure, e.g., to 40% of the hard-pressure.
 10. Anapparatus for continuous casting of slabs and ingots, in particular, ofthin slabs in a continuous casting installation including a softreduction section in a strand guide under a mold and having pressurerollers and support rollers (3,4) which are continuously restrainedrelative to one another, either individually or as a segment (1) byhydraulic cylinders (7,7′), and are restrained by stops (30) in alimiting manner, in particular for effecting the method according to theinvention, characterized by a hydraulic circuit with a pressure sensor(10) and a pressure changeover valve (11) operated by it, a shut-offvalve (12) arranged in its switching circuit, a control valve (13) and arelief valve, a pipe-break preventing means including a pressure reliefvalve (15) with a check valve for providing a hard-pressure restrainingforce within the not yet completely solidified region of the softreduction section or a soft-pressure restraining force within thecompletely solidified region of the soft reduction section, and forswitching from the hard pressure to the soft pressure and vice versa.